Allison Williams is a million miles away from her neurotic, uptight character in Lena Dunham's hit HBO comedy Girls. Instead, the 26-year-old is chatty, calm, funny and a whole lot more sorted and settled than her on-screen counterpart.

Not that it should come as much of a surprise. Since the first series of Girls aired in America in 2012, Williams' star has been on the rise. Plucked form relative obscurity by comedy king Judd Apatow (after he watched her singing along to the Mad Men theme tune, no less) Williams is now a common sight on the covers of women's glossies. And with Girls steaming on ahead for a fourth series, life looks as though it can only get better for the daughter of NBC news anchor Brian Williams.

"It’s the best job in the world," Williams tells us at the UK launch for the show's third series. "It would be good enough to play someone for this many years on HBO written by Lena Dunham. But the fact that Marnie has evolved so much since I’ve been playing her really does mean that, yes, I’m playing one person but I’ve played so many versions of that one person. I’ve played her up and down and sideways and confused and upset and happy and nervous and comfortable. I’ve played her in every different situation imaginable against so many different romantic guys, against so many friends... That’s been such a pleasure and it makes what could be the monotony of television totally not monotonous and completely unexpected, fun and unpredictable. Every day is different."

So, naturally, that made the decision to return for another series an easy one for the actress. "Oh my gosh, I never have to think about it," says Williams. "I will be Marnie for as long as they will have me."

That's not to say that working on the set of Girls is an easy ride, though (no pun intended...). The show, which also stars Zosia Mamet, Jemima Kirke and Dunham herself, is notorious for its long, awkward sex scenes – and none of the cast are exempt from stripping off and having their moment in the limelight.

"I assume at some point I’ll be in a movie where there’s like a very sort of breathy, airy sex scene that’s supposed to be beautiful and erotic and whatever – and it will feel so weird to me," says Williams.

"There's a lot of integrity [in Girls' sex scenes]. Lena was recently speaking about how she wants the sex scenes to normalise sex for other people. She wants people to see the sex on our show and go 'Oh my God, other people have sex like that!'

"That can be why it's so uncomfortable to watch the show with other people because you kind of have to dip your toe in and say to your girlfriends, 'Has anyone ever had sex like that?' And then it's a unanimous 'Yes!' and you're like 'Phew. I'm not the only one here!'"

Normalising sex isn't the only thing Williams reckons Girls can achieve. The show also opens up discussion about relationships by giving women "a bunch of templates they can use."

"I have friends who have been with 'Adams' since the show's been on air and it's so much easier to begin to explain it that way!" she laughs.

Back to those bedroom encounters themselves, though, and Williams describes the atmosphere on set when filming them as like being in high school. "I don’t care how old you are, that’s always how it’s going to be. No one's cool in that situation. You just giggle. There’s kind of no way around it."

"The most fun sex scene ever was with Andrew Rannells," she continues. "We laughed so hard throughout the entire scene I was shocked they were able to find enough footage to cut together to make the scene... My nipple covers kept flying off and his thing kept falling... It was hysterical."

"I’d never done one previous to this show and now I was Andrew’s first sex scene partner and I was able to like walk him through it. I was very gentle with him. I take great pride in taking people’s sex scene virginity."

When it comes to the plot lines themselves, Dunham is well known for adding her real life experiences to the show's script. So does Williams ever worry that her bad days or embarrassing experiences will one day be broadcast around the world?

"It’s funny, a friend of mine has this saying that when a day is going terribly that it’s smart to sort of step back and be like, this is a good episode of my life. This might even be a season finale – it’s a dramatic day! I definitely try to keep that in mind and keep that sense of levity.

"Lena’s pretty generous about not mining our lives without our permission for the show, [so] I always watch what’s happening in Lena’s life and I wonder if this is going to show up on the show... But for the most part she uses her imagination in a way that’s – I don’t have to say it for the millionth time – absolutely brilliant. She can pull this out of the sky and make it seem like she’s lived each of these experiences even if she hasn’t even come close."